Plantar tissue stiffness in patients with diabetes mellitus and peripheral neuropathy.

OBJECTIVE To determine if a difference exists in the plantar soft tissue of patients with diabetes mellitus (DM) and peripheral neuropathy (PN) compared with age-matched controls. DESIGN Case-control study with a parallel 3-element 1-dimensional viscoelastic model developed to characterize indentation data. SETTING Data collection performed in an academic physical therapy laboratory. PARTICIPANTS Forty subjects were recruited into 2 groups (20 subjects with DM, PN, and history of plantar ulcers; 20 control subjects), matched for age (DM: 55.22+/-9.39 y; control: 55.91+/-10.97 y), gender (DM: 14 men, 6 women; control: 14 men, 6 women), and body mass index (DM: 32.96+/-8.39 kg/m(2); control: 32.58+/-7.69 kg/m(2)). INTERVENTIONS The plantar soft tissue stiffness was measured over the first, third, and fifth metatarsals, and heel of each subject using an indentor system that accurately measures force/displacement (F/D) data. A parallel 3-element viscoelastic mechanical model was then used to transform the F/D data into values that were used to make stiffness assessments. MAIN OUTCOME MEASURE The element coefficients of our model indicated the stiffness of the plantar tissue. RESULTS The plantar tissue of the subjects with DM over the metatarsal heads was stiffer than the control population as indicated by one of the spring constants in the parallel 3-element model (first: 1.13+/-0.55 N/mm vs.72+/-.32 N/mm; third:.96+/-.32 N/mm vs.79+/-.17 N/mm; fifth:.90+/-.31 N/mm vs.69+/-.28 N/mm; P<.05). CONCLUSIONS The plantar tissue of subjects with DM, PN, and a history of ulcers was stiffer than control subjects. However, additional research is needed to determine the relationship among increased soft tissue stiffness, plantar pressures, and skin breakdown.